1. Field of the Invention
The present invention relates to a switching power supply, and more specifically, a switching power supply which obtains a DC output power from an AC input power and which has a function of improving the power factor of an AC input current.
2. Description of the Related Art
In order to obtain a DC power from a commercially available power source (an AC power) through rectification and smoothing, a configuration of using a diode bridge and a smoothing capacitor is the simplest configuration. According to this configuration, however, the circuit configuration becomes a so-called capacitor-input type rectification circuit which has an input current allowed to flow only near the peak of a power-supply voltage. Accordingly, the power factor is reduced and the input harmonic increases, so that the problem of the input harmonic is regulated by an international standard. In order to clear such a standard, a converter is proposed which is called a PFC (Power Factor Correction) converter or a high power factor converter.
Among such converters, the most common circuit configuration is a so-called boosting PFC converter. Such a converter has a series circuit of a coil and a switch connected between the positive side and the negative side of a diode bridge that rectifies an AC current, the anode of a booster diode is connected to a contact between the coil and the switch, the cathode of the booster diode is connected to the high-voltage side of an output smoothing capacitor, and the low-voltage side of the output smoothing capacitor is connected to the negative side of the diode bridge.
A report EE2002-83 by the institute of electronics, communication engineers discloses a switching power supply which has a converter like a single-stage type active clamping power factor correction converter with a PFC function, an insulation function, and an output-voltage stabilizing function. This switching power supply uses an active clamping flyback converter as a base, has a coil at the DC side of a diode bridge that rectifies a commercially available AC current, and improves the power factor by causing the current of the coil to operate in a discontinuous mode. In addition, various kinds of circuits are proposed for an insulating converter with a PFC function.
Also, JP 2008-306927 A discloses a technology which has a third winding and which supplies an output power by the third winding as an energy source for the primary side to the control circuit of a switching device connected in series to the primary winding of a transformer.
The most common circuit configuration, so-called a booster PFC converter, however, has no insulation function, and needs an insulation DC/DC converter having an insulation transformer and connected at the following stage of the PFC converter in order to obtain a voltage, such as a DC 24 V or a DC 12 V since it is a booster type, and thus obtaining a desired DC voltage. Hence, according to this configuration, a conversion circuit intervenes in order to obtain a DC voltage, so that the total conversion efficiency is poor and has a technical issue from the standpoint of energy saving.
Also, the circuit configuration disclosed in a report EE2002-83 employs a two-stage converter configuration, so that the conversion efficiency of power is a multiplication of the efficiencies of the two-stage converters, and thus the total conversion efficiency decreases.
Also, the technology disclosed in JP 2008-306927 A is insufficient for stabilization of the whole power efficiency and output voltage.
As explained above, according to the prior art, an applied voltage at the time of turn-off of a switching device that performs switching is high, the whole power efficiency is low, and the harmonic suppressing function and the instant power-suspension compensating function are insufficient, so that a capacitor with a large capacity is required, and thus a switching power supply becomes large and thick in size.